docs: def check_time_period(self, ds):
          """
          Check that time period attributes are both set.
          """
          start = self.std_check(ds, 'time_coverage_start')
          end = self.std_check(ds, 'time_coverage_end')
  
          msgs = []
          count = 2
          if not start:
              count -= 1
              msgs.append("Attr 'time_coverage_start' is missing")
          if not end:
              count -= 1
              msgs.append("Attr 'time_coverage_end' is missing")
  
          return Result(BaseCheck.HIGH, (count, 2), 'time coverage start/end', msgs)